Before you can properly understand the effect added ingredients have on your bread, it's important to know how yeast breads work. Bread gets its structure and texture from gluten, long and stretchy strands of protein created when the flour is moistened and worked into dough. As the yeasts in your dough go about their business, they leave behind tiny amounts of carbon dioxide as waste. The rubbery strands of gluten trap those gas bubbles, which then blow up like tiny balloons as the carbon dioxide expands in the oven's heat. The flour's proteins become firm as your bread bakes, just as an egg solidifies in your frying pan, and give the loaf its final shape.

A few kinds of bread rely on highly flavored oils, such as virgin olive oil, for their flavor. Aside from those, oil is usually added to bread to make it softer. You might enjoy the tough, chewy crust of an artisanal loaf occasionally, but sometimes a softer result is more versatile. Oil serves that purpose. It lubricates the flour's proteins, shortening the strands of gluten and weakening the bonds that hold them together. That makes the bread softer and easier to chew. Vegetable oil shortening serves the same purpose, which is how it acquired its name.

Eggs have a more complicated effect on bread, because they're a more complicated ingredient. They consist largely of water, so they add to the bread's liquids and provide some of the moisture that helps gluten develop. The yolks also contain a high percentage of fat, which like oil lubricates the proteins and weakens gluten development. However, eggs also contain large quantities of protein. Those proteins set and become firm when the bread bakes, helping contribute to its crumb structure. Egg proteins are softer and less chewy than gluten proteins, so the end result is a loaf with a tenderer texture.

Those attributes alone would make eggs a noteworthy bread ingredient, but they have a handful of other useful effects as well. They add richness and flavor to the bread, so sweet breads and festive breads almost always include eggs. They also contain powerful emulsifiers that help bring fat-based ingredients and water-based ingredients together into a smooth, pliant dough. That's especially important in butter-rich sweet or holiday breads, a second reason those loaves almost invariably include eggs. Eggs also give the loaf a rich golden color inside and out, and many bakers brush beaten egg over the loaves before baking to intensify the effect.